Exhaust gas treatment unit and measuring instrument for ascertaining a concentration of a urea-water solution

ABSTRACT

An exhaust gas treatment unit for conversion and in particular reduction of nitrogen oxide compounds in an exhaust gas stream of a combustion system, in particular a Diesel internal combustion engine, having a storage unit for storing a urea-water solution and a metering unit for metering the urea-water solution into the exhaust gas stream, and at least one measuring instrument for ascertaining a concentration of the urea-water solution in the storage unit with an electronic evaluation unit with which the concentration of the urea-water solution can be ascertained without major effort and economically favorably, so that an exact metering of urea into the exhaust gas stream can be achieved. At least one measuring instrument is provided in the treatment unit, for ascertaining a concentration of the urea-water solution in the storage unit with an electronic evaluation unit, and the storage unit of the measuring instrument includes at least one pressure sensor for ascertaining a pressure.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to an exhaust gas treatment unit for converting an exhaust gas stream in a combustion system, and to a measuring instrument for ascertaining a concentration of a urea-water solution in a storage unit, as generically defined by the preambles to claims 1 and 11, respectively.

[0003] 2. Description of the Prior Art

[0004] Especially in conjunction with future regulations for nitrogen oxide emissions for motor vehicles, suitable exhaust gas treatment cannot be avoided. To reduce NO_(x) emissions from internal combustion engines, for instance not only gasoline engines but above all Diesel engines, the so-called selective catalytic reduction (SCR) process is to be employed, among others. In it, a defined quantity of reducing agent in the form of urea or urea-water solution is added to the exhaust system (see WEISSWEILER, in CIT (72), 441-449, 2000).

[0005] For the stoichiometric conversion of the ammonia (NH₃), generated from urea ((NH₂)₂CO), and the nitrogen oxides (NO_(x)) present in the exhaust gas, the urea-water solution is metered into the exhaust system of the engine. This means that in a first reaction stage, the urea contained in the urea-water solution, particularly in the exhaust system, is hydrolized with water to form ammonia and carbon dioxide. In a second reaction stage, the ammonia reduces the nitrogen oxides contained in the exhaust system to nitrogen, the byproduct being water, in accordance with the following complete reaction:

[0006] NO+NO₂+2NH₃→2N₂+3H₂O.

[0007] In exhaust gas posttreatment systems at present for NO_(x) reduction, however, it is disadvantageous that urea in the urea-water solution automatically decomposes as a function of the temperature into ammonia (NH₃) and carbon dioxide (CO₂), which reduces the urea concentration in the urea-water solution. Accordingly, exact metering of the urea-water solution, which is stored for comparatively long periods of time, into the exhaust gas stream for stoichiometric conversion of the urea cannot be assured. With a reduced concentration of urea in the urea-water solution, correspondingly less NO_(x) is converted in the exhaust gas, since the product of decomposition, NH₃, escapes from the urea-water solution in the tank. As a result, undesirably high NO_(x) concentrations can be emitted.

[0008] An overmetering of urea-water solution into the exhaust gas to prevent undesirably high NO_(x) emissions might be conceivable. A disadvantage of this variant, however, is the release of ammonia that could occur under some circumstances, that is, the emission of unconverted, poisonous ammonia.

OBJECT AND SUMMARY OF THE INVENTION

[0009] The primary object of the invention is to provide an exhaust gas treatment unit for converting an exhaust gas stream in a combustion system, and a measuring instrument for ascertaining a concentration of a urea-water solution, with which the concentration of the urea-water solution can be ascertained without major effort and economically favorably, making more exact urea metering into the exhaust gas stream feasible.

[0010] This object is attained, based on an exhaust gas treatment unit and a measuring instrument of the type introduced above in which, in the exhaust gas treatment unit, at least one measuring instrument is provided for ascertaining a concentration of the urea-water solution in the storage unit with an electronic evaluation unit, and that the storage unit of the measuring instrument includes at least one pressure sensor for ascertaining a pressure.

[0011] According to the invention, the concentration of the urea-water solution can be ascertained without comparatively great effort, making virtually exact metering of the urea into the exhaust gas stream feasible, and hence making it maximally possible to prevent disadvantageously high emissions of NO_(x) and ammonia from corresponding combustion systems and vehicles.

[0012] In general, for the stoichiometric conversion of the urea-water solution and the exhaust gas stream, the NO_(x) emissions can advantageously be ascertained. For instance, by means of a management system that may already be present, in which NO_(x) emissions corresponding to them are associated with various operating states of the combustion unit.

[0013] Optionally, as an alternative or in combination with this, at least one exhaust gas sensor, with the aid of the evaluation unit, among others, can ascertain in particular the NO_(x) concentration in the exhaust gas stream.

[0014] With the aid of the pressure sensor of the invention, an indirect measuring method is employed for ascertaining the concentration of the urea-water solution in the storage unit. Advantageously, the evaluation unit, by means of the pressure sensor, ascertains the vapor pressure of the ammonia generated in the virtually gastight storage unit on the basis of the hydrolysis of the urea, plus the vapor pressure of the solvent, which is water, so that the evaluation unit can ascertain the accordingly reduced quantity or concentration of urea in the urea-water solution.

[0015] Measuring methods already known at present for ascertaining a concentration of a urea-water solution are performed by means of complicated laboratory analysis and suitable items of equipment, which are not feasible, especially for use in vehicle applications and because of peripheral conditions in terms of process technology and economy.

[0016] Preferably, commercially available pressure sensors are used, making an economically especially favorable realization of the invention possible. Above all, ascertaining the urea-water solution concentration economically favorably and relatively simply in terms of process technology makes it possible to use suitable measuring instruments in exhaust gas treatment units, especially for such vehicles as Diesel trucks or passenger cars. In view of the regulations already known now, which will be even more stringent in future, the use of measuring instruments of the invention is intended for provision above all for exhaust gas treatment in motor vehicles with Diesel engines.

[0017] In principle, however, a measuring instrument of the invention can also be used in exhaust gas treatment in other combustion systems, such as Otto engines or the like for converting or reducing the nitrogen oxide compounds contained in the corresponding exhaust gases; this might become necessary, if current regulations are made more stringent.

[0018] In a particular refinement of the invention, the evaluation unit includes at least one temperature sensor for ascertaining a temperature, especially of the urea-water solution. Above all in the range between −11° and +90° C., a certain quantity of urea decomposing in the urea-water solution corresponds to each temperature, so that by means of the temperature sensor and the evaluation unit, the correspondingly converted quantity can be ascertained, and from that the concentration of the urea-water solution can be ascertained comparatively accurately.

[0019] Moreover, by means of the temperature sensor, the dependency of the pressure in the storage unit on the current temperature can advantageously be taken into account and thus used in ascertaining the urea-water solution concentration. Consequently, pressure changes caused for instance by fluctuations in the outdoor temperature can accordingly be taken into account or associated with their cause. This provision additionally improves the accuracy of urea-water solution concentration determination and thus the metering of urea-water solution into the exhaust gas stream. Optionally, at least one further, separate temperature sensor for ascertaining the ambient temperature of the storage unit can be advantageous.

[0020] Preferably, a commercially available standard element is used as the temperature sensor. Optionally, combined pressure and temperature sensors, which are already on the market and are in the form of a structural unit can be employed in accordance with the invention. These provisions make an especially economically favorable embodiment possible.

[0021] Advantageously, the evaluation unit includes at least one timer for measuring the time. This assures that as a function of time, above all the pressure change or decomposition of the urea in the urea-water solution is ascertainable. Advantageously, commercially available components are employed as the timer, or a suitable component in an existing electronic unit in the evaluation unit is used, such as a suitable chip or the like.

[0022] By means of the pressure change or the pressure increase over time, it is advantageously possible to ascertain, among others, the actual initial concentration, that is, the urea-water solution concentration immediately after the storage unit is filled, or the urea-water solution concentration prevailing at the current time. Advantageously, it is also possible according to the invention to detect whether the solution in the storage unit is a urea-water solution, or whether there is no urea-water solution and for instance only pure water in the storage unit. For instance, by means of the ascertained pressure increase in the storage unit after approximately 1 hour at temperatures of approximately 40° C., a urea-water solution can be distinguished from virtually pure water.

[0023] In general, the “fresh” urea-water solution has a eutectic concentration of 32.5%. However, even partly decomposed urea-water solution at an unknown, lesser concentration can also be introduced into the storage unit and optionally mixed with an already decomposed urea-water solution.

[0024] The decomposition of the urea-water solution into ammonia and carbon dioxide is expressed, as already noted, by a pressure increase in the virtually gastight urea-water solution storage unit. However, since some of the ammonia produced is dissolved in water, or in other words aqueous NH₃ is formed, some of it decomposes in water, for instance into NH₄OH, and because the carbon dioxide component dissolved in the water affects the ammonia absorption performance of the water, in an advantageous variant of the invention, the evaluation unit has at least one electronically stored characteristic curve for temperature, pressure and/or time parameters. Corresponding characteristic curves are preferably made the basis of ascertaining the concentration by means of the evaluation unit.

[0025] In an advantageous embodiment of the invention, the evaluation unit has at least one volume measuring element, disposed on an outflow element, in particular a tailpipe or exhaust line, of the storage unit for measuring the volume of the urea-water solution flowing out of or removed from the storage unit. This makes it possible to determine the quantity of urea-water solution withdrawn from the storage unit, so that the resultant pressure drop in the storage unit can be ascertained and used, among other purposes, by means of the evaluation unit for advantageously determining the urea-water solution concentration in the storage unit.

[0026] Optionally, the volume measuring element can be embodied as a metering valve or the like. By means of a pump, for instance, the metering valve can be subjected to a virtually constant pressure, and by detecting the opening time of the metering valve using the timer, the virtually exact ascertainment of the volume withdrawn from the storage unit can be achieved.

[0027] Preferably, the volume measuring element is also used for maximally exact metering of the urea-water solution for the stoichiometric conversion together with the exhaust gas stream. This also lowers the engineering expense for corresponding exhaust gas treatment units.

[0028] Advantageously, the storage unit has at least one regulatable, closable opening for pressure compensation. This assures that impermissible pressure, whether overpressure or underpressure, in the storage unit cannot be generated as a result of the decomposition of urea-water solution or generation of ammonia, temperature change, and/or removable of urea-water solution.

[0029] Advantageously, the timer ascertains at least one duration of an opening phase of the opening, so that particularly in cooperation with the pressure sensor, the evaluation unit above all ascertains the outflowing or inflowing quantity of ammonia. The opening is advantageously disposed in the upper region of the storage unit, so that above all gaseous ammonia can flow out and/or atmospheric air can flow in.

[0030] In a particular variant of the invention, the opening includes at least one overpressure and/or underpressure valve, optionally a so-called combination valve, for pressure compensation of the urea-water solution storage unit. Advantageously, by means of appropriate valves, the pressure in the storage unit can be kept within predetermined limits, so that in a preferred way, the storage unit need not be embodied as a pressure vessel. For instance, approximately 100 mbar to 2 bar is specified as the maximum overpressure, and 20 to 100 mbar as the maximum underpressure. Accordingly, possibly disadvantageously high economic costs for pressure reservoirs that would otherwise have to be used can be avoided.

[0031] In a particular refinement of the invention, the storage unit has at least one fill level sensor for ascertaining a fill level. This makes it possible for the quantity of urea-water solution in the storage unit to be detected independently of use or of the quantity of urea-water solution withdrawn. This is advantageous for determining the quantity stored in the storage unit, especially after the storage unit has been refilled.

[0032] Moreover, especially by means of the fill level sensor, a warning signal can be generated for a suitable display unit or gauge if the storage unit is nearly empty. For instance, by means of a suitable display in a vehicle or the like, the operator can be made aware that the storage unit has to be refilled with urea-water solution.

[0033] As fill level sensors, conventional standard elements can advantageously be used. As an alternative, at least the pressure sensor is embodied as a fill level sensor, as a result of which, advantageously, a structurally and economically favorable embodiment of the invention can be achieved. Advantageously, the suitably embodied pressure sensor is disposed in the lower region of the storage unit, so that the fill level can be ascertained by means of the hydrostatic pressure of the urea-water solution.

[0034] In an advantageous variant of the invention, in an exhaust gas treatment unit, a flowthrough element, in particular a tubular element, connecting line or the like, for delivering a gaseous fluid, in particular ammonia, to the exhaust gas stream is disposed at least at the regulatable, closable opening of the storage unit. With the aid of this provision, it becomes possible for the poisonous ammonia generated to be used for reducing the nitrogen oxide compounds of the exhaust gas stream of the combustion system.

[0035] In principle, by means of the measuring instrument of the invention, at a given temperature the concentration of the urea can be ascertained from the current pressure of the storage unit; particularly in conjunction with the evaluation of these parameters by means of the evaluation unit, and optionally in interaction with a control or regulating unit of the exhaust gas treatment unit or vehicle, this assures substantially more-accurate metering of the urea-water solution into the exhaust gas stream.

BRIEF DESCRIPTION OF THE DRAWING

[0036] The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawing, in which

[0037]FIG. 1 shows one exemplary embodiment of an exhaust gas treatment unit with a measuring instrument in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] In FIG. 1, an exhaust gas treatment unit for NO_(x) reduction of a crude exhaust gas stream 1 in a combustion system, not shown in detail, such as a Diesel or gasoline engine or the like, is shown. The crude exhaust gas stream 1 is optionally pretreated in an oxidation catalytic converter 2. Preferably downstream in the flow direction of the oxidation catalytic converter 2, urea-water solution 5 is atomized largely uniformly by means of a nozzle 4 in the region of an SCR catalytic converter 3.

[0039] In a known manner not shown, in the SCR catalytic converter 3, in a first reaction stage, the urea ((NH₂)₂CO) contained in the urea-water solution 5 is converted or hydrolized with water (H₂O) to form ammonia (NH₃) and carbon dioxide (CO₂). In a second reaction stage, ammonia and in particular nitrogen monoxide (NO) as well as nitrogen dioxide (NO₂) are converted into nitrogen (N₂) and water.

[0040] In FIG. 1, both the hydrolysis and the reduction can be done in one structural unit 3. As an alternative, however, in a manner not shown in detail, one separate, preferably catalytically active structural unit can be provided for the hydrolysis on the one hand and for the reduction on the other.

[0041] In general, also in a manner not shown in detail, still other components such as (Diesel soot) particle filters or the like can be provided for the exhaust gas treatment of the crude exhaust gas stream 1.

[0042] For stoichiometric conversion of the urea-water solution 5 together with the NO_(x) emissions from the engine, the urea-water solution 5 stored in a tank 6 is delivered at pressure, such as 20 to 25 bar, by a pump 7 and a regulating valve 8 to a metering valve 9. For injecting the urea-water solution 5, an air stream 10, preferably from the atmosphere, is additionally delivered to the metering valve 9 by means of a pump 11 and a pressure reservoir 12 and a regulating valve 13. The atomization of urea-water solution and air into the exhaust gas stream 1 is achieved by attaining a largely uniform oncoming flow to the SCR catalytic converter 3. Optionally, advantageous flow elements such as baffles or the like can be provided for this purpose in the SCR catalytic converter 3.

[0043] For the hydrolysis of the urea-water solution 5, a minimum operating temperature of approximately 220° C. should be provided in particular, so that even independently of the present invention, to improve cold starting performance above all and/or in other special operating phases of the exhaust gas treatment unit, a heating unit, heat exchanger unit or the like can be disposed on the SCR catalytic converter 3 in a manner not shown. Optionally, waste heat from an exothermally functioning component of the exhaust gas treatment unit or from the engine or the like can be used, and/or preferably an electrical or catalytically active heating unit can be used for heating the SCR catalytic converter 3.

[0044] According to the invention, the concentration of urea in the urea-water solution 5 should be ascertained selectively. In the range between −11° and +90° C., each temperature is associated with a certain quantity of urea decomposing in the urea-water solution 5, so that particularly over several hours or days, the concentration of urea-water solution 5 in the tank 6 varies or is reduced decisively. This decomposition of the urea-water solution 5 into ammonia and carbon dioxide, that is, the current urea-water solution concentration, is ascertained according to the invention above all by means of a pressure sensor 14 in the tank 6.

[0045] The tank 6 is preferably closed largely in gastight fashion by means of suitable tank connection necks 18 or inlet and outlet openings. The pressure increase in the urea-water solution tank 6 is expressed by the vapor pressure of the resultant ammonia, plus the vapor pressure of the solvent, water. Since some of the ammonia produced is dissolved in the water, and some of it decomposes in the water, for instance to form NH₄OH, a simple reverse calculation, limited merely restricted to the reduced nitrogen components of urea and NH₃, is possible.

[0046] Moreover, the content of CO₂ dissolved in the water also affects the ammonia absorption of the water. Especially in preliminary tests, it is therefore ascertained what combination of temperature, pressure and time corresponds with what concentration of urea in the urea-water solution 5. This is made the basis, in the form of a characteristic curve or performance graph, of the concentration calculation by means of a control unit 15. To that end, the tank 6 additionally has a temperature sensor 16, and the control unit 15 includes a timer for measuring the time.

[0047] In a manner not shown, the tank 6 can include an overpressure and/or underpressure valve in the upper region, so that a pressure compensation can be accomplished within predetermined limits, for instance between 2 bar of overpressure and 20 mbar of underpressure. This is advantageous especially because the pressure in the tank 6 can vary relatively sharply, for instance upon heating or cooling in response to external temperature changes and/or the withdrawal of urea-water solution 5, and could accordingly cause an undesirably major pressure change. With the aid of the applicable pressure compensation, it becomes possible in particular not to embody the tank 6 as a pressure reservoir. This assures an economically favorable embodiment of the tank 6.

[0048] This pressure compensation opening, or the compensation valve, communicates in a manner not shown in detail with the exhaust system by means of a connecting line, so that during certain operating phases and while the vehicle is parked, the ammonia, in particular, for reducing the NO_(x) emissions can be introduced into the exhaust gas stream 1. Optionally, the switching times for the events of opening the pressure compensation opening will also have to be passed on to the control unit, to enable reverse calculation from the reduced or escaped pressure or volume to arrive at the urea-water solution concentration.

[0049] In FIG. 1, the tank 6 includes a fill level sensor 17, so that the fill level of urea-water solution 5 in the tank 6 can be ascertained. As a result, it is possible to ascertain the fill level independently of the withdrawal of urea-water solution 5 from the tank 6. The quantity withdrawn can optionally be ascertained by means of the fill level sensor 17.

[0050] In a manner not shown in detail, the pressure sensor 14 can be embodied as a fill level sensor 17. To that end, it is disposed in the lower region or the region of the bottom of the tank 6. Via a suitable calibration using the characteristic curve of the control unit 15, the fill level can thus be detected on the basis of the applicable hydrostatic pressure of the urea-water solution 5.

[0051] In a manner not shown in detail, the tank 6 can include at least one heater unit or heat exchanger unit, so that freezing of the urea-water solution 5 at temperatures of less than −11° C. can be prevented.

[0052] Alternatively or in combination with this, the tank 6 can, in a manner not shown in detail, include a recirculation unit for recirculating the urea-water solution 5 in the tank 6 in order to prevent it from freezing at temperatures of less than −11° C.

[0053] Optionally, the urea-water solution flow lines or components containing urea-water solution 5 that are shown in FIG. 1 can in particular be embodied as heatable electrically.

[0054] According to the invention, on the basis of comparatively accurate knowledge of the urea content in the urea-water solution 5, relatively exact metering of the urea-water solution 5 into the exhaust gas stream 1 can be achieved. This assures that by means of an exhaust gas stream 1, neither an undesirably high expulsion of NO_(x) nor an undesired ammonia slip into the environment will occur.

[0055] For monitoring or checking the NO_(x) reduction or the exhaust gas treatment, for instance, an exhaust gas sensor 20 for ascertaining NO_(x) or ammonia, and/or a temperature sensor 21, is optionally provided downstream in the flow direction from the SCR catalytic converter 3.

[0056] In particular, all the sensors 14, 16, 17, 20, 21 and the valves 8, 9, 13 are electrically connected to the control unit 15 for data transmission. Furthermore, the pump 7 and the pressure compensation valve, not shown, of the tank 6 also communicate with the control unit for control or regulating purposes. A so-called data bus system, such as a CAN bus or the like, is advantageously provided, for the sake of a favorable mode of operation of the exhaust gas treatment unit and/or of the applicable vehicle.

[0057] The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims. 

I claim:
 1. A measuring instrument for ascertaining a concentration of a urea-water solution (5) in a storage unit (6) comprising an electronic evaluation unit (15), the storage unit (6) including at least one pressure sensor (14) for ascertaining a pressure.
 2. The measuring instrument of claim 1 wherein the storage unit (6) further comprises at least one temperature sensor (16) for ascertaining a temperature.
 3. The measuring instrument of claim 1 wherein the evaluation unit (15) further comprises at least one timer for ascertaining the time.
 4. The measuring instrument of claim 2 wherein the evaluation unit (15) further comprises at least one timer for ascertaining the time.
 5. The measuring instrument of claim 1 wherein the evaluation unit (15) has an electronically stored characteristic curve of pressure.
 6. The measuring instrument of claim 2 wherein the evaluation unit (15) has an electronically stored characteristic curve of pressure, and curve of temperature.
 7. The measuring instrument of claim 4 wherein the evaluation unit (15) has an electronically stored characteristic curve of pressure, and curve of time.
 8. The measuring instrument of claim 1 wherein the evaluation unit (15) further comprises a volume measuring element (9), disposed on an outflow element of the storage unit (6), for measuring the volume of the urea-water solution (5) removed from the storage unit (6).
 9. The measuring instrument of claim 1 wherein the storage unit (6) has at least one regulatable, closable opening for pressure compensation.
 10. The measuring instrument of claim 9 wherein the elevation unit 15 further comprises at least one timer, and wherein the timer ascertains at least one duration of an opening phase of the opening.
 11. The measuring instrument of claim 9 wherein the opening includes at least one overpressure and/or underpressure valve.
 12. The measuring instrument of claim 1 wherein the storage unit (6) has at least one fill level sensor (17) for ascertaining a fill level.
 13. The measuring instrument of claim 1 wherein at least the pressure sensor (14) is embodied as a fill level sensor (17).
 14. An exhaust gas treatment unit for conversion and in particular reduction of nitrogen oxide compounds in an exhaust gas stream (1) of a combustion system, in particular a Diesel internal combustion engine, comprising a storage unit (6) for storing a urea-water solution (5) a metering unit (7, 8, 9, 10, 11, 12, 13, 15) for metering the urea-water solution (5) into the exhaust gas stream (1), an electronic evaluation unit (15), and at least one measuring instrument for ascertaining a concentration of the urea-water solution (5) in the storage unit (6) with the electronic evaluation unit (15).
 15. The exhaust gas treatment unit of claim 14 wherein the measuring instrument comprises at least one pressure sensor (14) for ascertaining a pressure.
 16. The exhaust gas treatment unit of claim 14 wherein the measuring instrument comprises at least one pressure sensor (14) for ascertaining a pressure, but at least one temperature sensor for ascertaining a temperature.
 17. The exhaust gas treatment unit of claim 14 wherein the measuring instrument comprises at least one pressure sensor (14) for ascertaining a pressure, but at least one timer for ascertaining the time.
 18. The exhaust gas treatment unit of claim 14 further comprising a flowthrough element for delivering a gaseous fluid to the exhaust gas stream (1) disposed at a regulatable, closable opening of the storage unit (6).
 19. A vehicle having an exhaust gas treatment unit for conversion and in particular reduction of nitrogen oxide compounds in an exhaust gas stream (1) of a combustion system, in particular a Diesel internal combustion engine, wherein the exhaust gas treatment unit is the exhaust gas treatment unit defined in claim
 14. 20. A method for ascertaining a concentration of a urea-water solution (5) in a storage unit (6) with an electronic evaluation unit (15), and at least one measuring instrument in the storage unit (6). 